The incident was reported at a building on Decatur Avenue just before 5:30 p.m.
The FDNY said initial readings were reported to be about 500 ppm. According to Kidde Fire Safety, levels greater than 101 ppm are considered to be dangerous.
Residents were evacuated from the building and firefighters mitigated the incident and got it under control by shutting down the boiler and ventilating the building.
Seven residents and four firefighters were taken to Jacobi Medical Center for possible carbon monoxide poisoning.
"Thank goodness the firefighters came when they did because they had to remove two people from the apartment that's directly across from mine, they had to kick down the door," resident Onesemo Pena said.
Con Ed said the incident started with a leak in the chimney stack.
DOB inspectors from the Boiler Unit responded to the scene to investigate.
"Our investigation has determined that the chimney for the boiler was blocked with debris, potentially causing carbon monoxide to back up into the boiler room," the DOB said.
The DOB has issued a Cease Use Order for the boiler and the property owner will need to make repairs in order for the boiler to be turned back on.
The conditions of the victims taken to the hospital are not known, but they are all expected to survive.
Officials say there were working carbon monoxide detectors in the building.